1. CSE1301 Sem
Selection
Lecture 25
Lecture 9: Selection

2. Summary of previous lecture
In the previous lecture we have been covered,
Algorithms and Programs
A C programming language
History of C
C, A High level language
How to get started with C.
Basic Structure of a C program
Data Storage and Data Types
Variables, Keywords, identifiers, Assignment

3. Summary of previous lecture
constant variable
printf() and scanf() functions and usage
Precedence
int and float
Unary operations
Increment and decrement operations
Comments
Error and its Types

4. Today’s Topics The if statement The else statement Cascaded if
Nested if
Switch statement

5. Conditions We come across conditions in daily life, For example
If you will get less than 50 marks then you will fail.
If team scored less than 250 runs, it will be out of the tournament.
If the temperature is greater than 50 degree Celsius then wear light dress.

6. Conditions in C Programming
Same if structure is used in c programming to test the condition.
It has two parts
Conditional or antecedent or LHS part and
Conclusion or action or consequent or RHS part.

7. The if statement if ( expression ) statement
Determines whether a statement or block is executed.
Implements the selection instructions within an algorithm.
Decides what to do by evaluating a Boolean expression.
If the expression is true (non-zero), the statement or block is executed.
if ( expression )
statement

8. What is a statement? An empty statement is a single semicolon.
Statements are lines of instructions in our programs ending with a semicolon (;).
A compound statement or block is a series of statements surrounded by braces.
E.g. {
number = number + 1;
printf("%d\n", number); }
An empty statement is a single semicolon.

9. Example: oddnum.c Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

10. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
return 0; }
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

11. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
return 0; }
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

12. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

13. Example: oddnum.c Do not put “then” here! #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }
Do not put “then” here!

14. Do not put semicolon here!
Example: oddnum.c
#include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }
Do not put semicolon here!

15. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

16. Notes on if
Which of the following code fragments are equivalent? A
if (number % 2 != 0) {
printf("%d", number); }
printf(” is odd\n"); B
if (number % 2 != 0)
printf("%d", number);
printf(” is odd\n"); C
if (number % 2 != 0) {
printf("%d", number);
printf(” is odd\n"); }

17. Notes on if
Which of the following code fragments are equivalent? A
if (number % 2 != 0) {
printf("%d", number); }
printf(” is odd\n"); B
if (number % 2 != 0)
printf("%d", number);
printf(” is odd\n"); C
if (number % 2 != 0) {
printf("%d", number);
printf(” is odd\n"); }

18. Notes on if
Which of the following code fragments are equivalent? A
if (number % 2 != 0) {
printf("%d", number); }
printf(” is odd\n");
A Compound Statement B
if (number % 2 != 0)
printf("%d", number);
printf(” is odd\n");
A Statement C
if (number % 2 != 0) {
printf("%d", number);
printf(” is odd\n"); }

19. Notes on if Common mistake if (number % 2 != 0); {
printf("%d is an odd ", number); }
printf("number\n");

20. Notes on if Common mistake The semicolon is an empty statement.
if (number % 2 != 0); {
printf("%d is an odd ", number); }
printf("number\n");
No semi-colon here!
The semicolon is an empty statement.

21. Notes on if
Common mistake
if (number = 0) {
printf("%d\n", number); }

22. Notes on if Common mistake if (number = 0) { printf("%d\n", number); }
Should be ==

23. The else statement Can only occur after an if statement
Is only executed when the if block does not execute
if ( expression )
statement1
else
statement2

24. Example: oddnum.c #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
Example: oddnum.c
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”

25. Example: oddeven.c #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
else
printf("%d is an even number\n",
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”

26. Example: oddeven.c No semicolons here! #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
else
printf("%d is an even number\n",
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”
No semicolons here!

27. Example: oddeven.c #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
else
printf("%d is an even number\n",
Example: oddeven.c
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”

29. Output of a program

30. Cascaded if statement
Multiple alternative blocks each with a Boolean expression.
First expression which evaluates to true causes execution of the associated block.
At most only one block will be executed.

31. Example: months.c Determine the number of days in a given month:
30 days = September,
April, June and November.
All the rest have 31,
Excepting February alone,
Which have 28 days clear,
And 29 in each leap year.
output “Enter an integer”
input month
if (month is September,
or April,
or June,
or November)
then {
output “30 days” }
else if (month is February)
output “28 or 29 days”
else
output “31 days”

32. Example: months.c int main() { return 0; } #include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days = September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

33. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
return 0; }
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days = September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

34. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

35. if (month==September || April || June || November )
Example: months.c
int main() {
int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;
Common mistake:
if (month==September || April || June || November )

36. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

37. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
else
printf("31 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

38. “Default” block. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
else
printf("31 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;
“Default” block.

39. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
else
printf("31 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

40. Q: Notes on Cascaded if What is the output if: letter is equal to ‘b’
if (letter >= ’a’) {
printf(“S1\n”); }
else if (letter <= ’z’)
printf(“S2\n”);
else if (letter >= ’A’)
printf(“S3\n”);
else if (letter <= ’Z’)
printf(“S4\n”); Q:
What is the output if:
letter is equal to ‘b’
letter is equal to ‘z’
letter is equal to ‘A’
letter is equal to ‘X’

41. A: Notes on Cascaded if What is the output if: letter is equal to ‘b’
Answer=S1
letter is equal to ‘z’
letter is equal to ‘A’
Answer=S2
letter is equal to ‘X’
Answer = S2
if (letter >= ’a’) {
printf(“S1\n”); }
else if (letter <= ’z’)
printf(“S2\n”);
else if (letter >= ’A’)
printf(“S3\n”);
else if (letter <= ’Z’)
printf(“S4\n”);

42. More Examples if (ch >= ’a’ && ch <= ’z’) {
printf(“%c is in lower case.\n”, ch); }
else if (ch >= ’A’ && ch <= ’Z’)
printf(“%c is in upper case.\n”. ch);
else if (ch >= ’0’ && ch <= ’9’)
printf(“%c is a digit with value %d.\n”, ch, ch - ’0’);

43. Example: Nested if if ( coldWeather ) { wearJumper = 1;
wearRaincoat = wearJacket = wearThermal = 0;
if ( raining )
wearRaincoat = 1;
else
wearJacket = 1;
if ( belowZero )
wearThermal = 1; }

44. Example: Nested if if ( coldWeather ) { wearJumper = 1;
wearRaincoat = wearJacket = wearThermal = 0;
if ( raining )
wearRaincoat = 1;
else
wearJacket = 1;
if ( belowZero )
wearThermal = 1; }

45. Example: Nested if if ( coldWeather ) { wearJumper = 1;
wearRaincoat = wearJacket = wearThermal = 0;
if ( raining )
wearRaincoat = 1;
else
wearJacket = 1;
if ( belowZero )
wearThermal = 1; }

46. Example: Nested if if ( coldWeather ) { wearJumper = 1;
wearRaincoat = wearJacket = wearThermal = 0;
if ( raining )
wearRaincoat = 1;
else
wearJacket = 1;
if ( belowZero )
wearThermal = 1; }

47. If conditions summary The if statement The else statement Cascaded if
Nested if
Common mistakes

48. Switch Statement

49. Multi-way Selection
In addition to two-way selection (if else), most programming languages provide another selection concept known as multi-way selection.
Multi-way selection chooses among several alternatives. C programming can use switch statement for multi-way selection.

51. The Switch statement
Expression here is user
entered variable value

52. The Switch statement
case is a default word.

53. The Switch statement Expression is matched with the constant of each
case, wherever it is
matched, next statements
are executed
The Switch statement

54. If the expression doesn’t
The Switch statement
If the expression doesn’t
matched with any case
default is executed.

55. Closed switch resembles
executed case

56. Switch statement

57. printFlag is a variable

59. In order to stop next case to be automatically executed, break
statement has been used.

60. Multiple cases being executed together

61. Summary of switch Statement Rules

62. statements separately.
Develop the program for assigning the grades using if and switch
statements separately.

63. #include<stdio.h> int main() { int marks;
printf("%s", "Enter yours Marks");
scanf("%d",&marks);
switch(marks)
case 100:case 99:case 98:case 97:case 96:case 95:case 94:
case 93:case 92:case 91:case 90:
printf("%s", "You got A grade");
break;
Switch statement
Remember you need constants
here, so explicitly write
all statements for which this
case may be executed.

64. case 89:case 88:case 87:case 86:case 85:case 84:
printf("%s", "You got B grade");
break;
case 79:case 78:case 77:case 76:case 75:case 74:
case 73:case 72:case 71:case 70:
printf("%s", "You got C grade");
case 69:case 68:case 67:case 66:case 65:case 64:
case 63:case 62:case 61:case 60:
printf("%s", "You got D grade");
In case any of these cases execute, control from case statement will be returned.

65. If user enters some wrong value default will be executed!
printf("%s", "You got F grade");
break; }
return 0;
If user enters some wrong value default will be executed!

66. Alternative If statements

67. First Check, if true rest of the statements will not be executed
#include<stdio.h>
int main() {
int marks;
printf("%s", "Enter yours Marks");
scanf("%d",&marks);
if (marks >=90)
printf("%s", "you got A grade"); }
else if (marks >=80)
User inputs marks
First Check, if true rest of the statements will not be executed
If first check is false, this statement will be checked and so on!

68. Logical comparison in if statement, which is not permitted in case{
printf("%s", "you got B grade"); }
else if (marks >=70)
printf("%s", "you got c grade");
else if (marks >=60)
printf("%s", "you got D grade");
Logical comparison in if statement, which is not permitted in case

69. Integer value is returned by this program!
else
printf("%s", "you got F grade");
return 0; }
Integer value is returned by this program!

70. Summary
If conditions are used to execute statement or a group of statements based upon logically tested values.
Alternative to if condition is switch statement.
Both if and switch statements form logical structure of a program.